The invention is directed to a method for making an optical fiber having optical properties that systematically vary along its length. This method is particularly useful for making dispersion managed (DM) single-mode optical waveguide fibers.
The recent advent of wavelength division multiplexing and amplifiers has increased system requirements to lower the dispersion and dispersion slope of the optical fiber. Several unique methods of making dispersion managed fiber have previously been disclosed that address these properties nicely. See, for example, U.S. patent application Ser. No. 08/844,997 (Berkey et al.) filed Apr. 23, 1997, and U.S. patent application Ser. No. 08/584,868, filed Jan. 11, 1996, now U.S. Pat. No. 5,894,537, the specifications of which are all hereby incorporated by reference.
Many of the methods to date have been relatively complex and therefore may involve higher cost than more standard methods of manufacture because of this complexity. It would be desirable to develop an alternative, easier method to manufacture optical fiber whose dispersion characteristics vary between positive and negative along the longitudinal direction of the optical fiber, particularly in the 1550 nm operating window.
One aspect of the present invention relates to an optical fiber having different diameters along its length, and a method of making such fiber. The index of refraction profile of the optical fiber preform is selected so that, when the optical fiber preform is drawn into an optical fiber having such differing diameters along its length, the result is an optical fiber which varies along its longitudinal length (i.e., corresponding to the regions of differing diameters) between regions of negative and positive dispersion in the 1550 nm operating window, which preferably consists at least of the window between about 1480 and about 1625 nm. In some embodiments, the fiber also varies between regions of negative and positive dispersion slope along the length of the fiber in the 1550 nm operating window, or additionally or alternatively maintains a very low dispersion slope along the length of the fiber. Fibers made in accordance with this method are excellent candidates for dispersion managed fiber applications.
Modern feedback control loops can be used to control both downfeed rate and draw rate to control fiber diameters. The fiber O.D. change is most quickly achieved by changing the tractor (fiber take-up) speed and thus the draw rate. As a result, the diameter of the core of the fiber changes as the tractor speed changes, thereby enabling the transition region between different diameters to be kept relatively short. In preferred embodiments, the fiber is drawn so that the segments of different diameters differ in magnitude of outside fiber diameter by greater than 3 microns, more preferably greater than 5 microns, and most preferably greater than 10 microns measured at the outside diameter of the fiber. Also, the fiber is preferably alternates between sections which are between 100 m and 3 km in length, and more preferably the alternating sections are least 250 m in length and less than 2 km.
Not just any refractive index profile can be employed to produce a fiber having such varying negative and positive dispersion characteristics along its length. For example, standard single mode fiber changes dispersion very little with diameter, particularly at 1550 nm. One preferred family of refractive index profiles which enables a fiber having the desired alternating dispersion characteristics, when drawn to differing diameters along its length, consists of a core region surrounded by a cladding region, wherein the core region comprises a central core region which is updoped with respect to said cladding region, the central core region is surrounded by a moat region which is downdoped with respect to said cladding region, and the moat region is surrounded by an annular ring region which is updoped with respect to said cladding region. Such profiles include those wherein the central updoped segment has an index of refraction delta percent between about +0.5 to 1.5, the depressed moat core region which surrounds the central core region has a delta percent in the range of about xe2x88x920.1 to xe2x88x920.7, and the updoped annular ring has a delta percent between about 0.1 to 1.0. The radii of the three segments (measured from the centerline of the fiber to the extrapolated intersection of the segment refractive index profile with the x-axis, the x-axis being equal to the index of refraction of the cladding layer) is selected so that, if the radius of the first centerline up-doped segment is taken to be a, the radius of the moat section taken to be b, then b/a preferably is between about 1.5 to 3.0, and if the outer radius of the optional annular ring is c, then c/a is between about 2.5 and 3.7. More preferred radius and delta percent values for such profiles will be discussed further below.
The result is a fiber which can be made to vary along its length between regions of negative and positive dispersion, yet has a net dispersion and dispersion slope which are both relatively low. Preferred fibers made in accordance in the present invention can be designed to alternate between local positive and negative dispersions having a magnitude at 1550 between 1.5 and 20 ps/nm-km, yet yield a net dispersion of less than 1.0 ps/nm-km, more preferably less than 0.5 ps/nm-km, and most preferably less than 0.1 ps/nm-km at 1550 nm. Preferred fibers made in accordance in the present invention yield a dispersion slope of less than 0.03 ps/nm2-km, more preferably less than 0.01 ps/nm2-km, and most preferably less than 0.005 ps/nm2-km over the wavelength range 1480 to 1625 nm.
Another aspect of the present invention relates to an optical fiber formed from a family of index of refraction profiles which can be made to exhibit a very low dispersion and, perhaps more importantly, a very low dispersion slope, in the 1550 nm operating window. This preferred family of index of refraction profiles is capable of achieving a wide variety of magnitudes of dispersion, yet at the same time extremely low dispersion slopes can be achieved.
Consequently, this particular profile is capable of yielding very useful dispersion managed fiber products made in accordance with the invention described above, i.e., by providing varying diameters along its length, with the result that the dispersion slope of the fiber is still maintained at a very low level. The index of refraction profile of the optical fiber preform is selected so that, when the optical fiber preform is drawn into an optical fiber having such differing diameters along its length, the result is an optical fiber which varies along its longitudinal length (i.e., corresponding to the regions of differing diameters) between regions of negative and positive dispersion in the 1550 nm operating window, which preferably consists of the window between about 1480 and about 1625 nm. Because this family of profiles is capable of achieving a wide variety of magnitudes of dispersion, yet at the same time exhibiting extremely low dispersion slopes, this family of profiles is particularly preferred for making fibers having alternating diameters, and consequently alternating dispersion characteristics, along their length. Such fibers made in accordance in the present invention can be made to have a wide variety of alternating positive and negative dispersion values in the 1550 nm operating window, yet the net dispersion along the entire length of the fiber is still maintained relatively low. Likewise, the dispersion slope can be maintained at a low value, i.e., less than 0.03 ps/nm2-km, more preferably less than 0.01 ps/nm2-km, and most preferably less than 0.005 ps/nm2-km over the wavelength range 1480 to 1625 nm. To obtain the most preferred low slope properties with this preferred family of profiles, in addition to the refractive index versus radius relationships disclosed above, the ratio of the index delta percents of the up-doped annular ring segment to that of the centerline up-doped segment is between about 0.4 to 1.1, more preferably between about 0.6 to 0.9.
Alternatively, this unique preferred family of index of refraction profiles is valuable for use in the manufacture of more conventional optical fibers which have unitary diameter, dispersion and dispersion slope characteristics along their length. For example, optical fibers have been made which have a uniform refractive index profile (and uniform core and clad diameters) along the axial length of the fiber, and yield a local dispersion of more than 1.0 ps/nm-km, as well as a dispersion slope of less than 0.03 ps/nm2-km, more preferably less than 0.01 ps/nm2-km, and most preferably less than 0.005 ps/nm2-km over the wavelength range 1480 to 1625 nm. Such properties are particularly advantageous for use in wavelength division multiplexing applications. Again, in order to obtain the most preferred low slope properties in this family of refractive index profiles, in addition to the refractive index versus radius relationships disclosed above, the ratio of the index delta percents of the up-doped annular ring segment to that of the centerline up-doped segment is between about 0.4 to 1.1, more preferably between about 0.6 to 1.0.
Additional features and advantages of the invention will be set forth in the detailed description which follows, and in part will be readily apparent to those skilled in the art from that description or recognized by practicing the invention as described herein, including the detailed description which follows, the claims, as well as the appended drawings.
It is to be understood that both the foregoing general description and the following detailed description are merely exemplary of the invention, and are intended to provide an overview or framework for understanding the nature and character of the invention as it is claimed. The accompanying drawings are included to provide a further understanding of the invention, and are incorporated in and constitute a part of this specification. The drawings illustrate various embodiments of the invention, and together with the description serve to explain the principles and operation of the invention.